Liquid nitrogen production



w. NELSON ErAL LIQUID NITROGEN PRODUCTION Filed Sept. 2. 1965 Sept. 2,6, 1967 United States Patent() Dany Filed Sept. 2, 1965, Ser. No. 484,604 Claims priority, application Canada, Dec. 16, 1964,

s claims. icl. 62-9) This invention relates to the liquefaction of a gas utilizing a liquid regrigerant of a type having a boiling point at some convenient pressure lower than the critical ternperature 4of said gas. In particular it concerns the production of liquid nitrogen while regasifying liquid natural gas thereby to recover a part of the refrigeration potential inherent in the liquid natural gas.

In the utilization of natural gas, it has become common practice to transport and store such gas in liquid form becauseV of the greatly reduced volume and economy in storage space achieved thereby. This procedure involves liquefying natural gas prior to transportation and storage and then regasifying the liquid prior to its delivery to a pipeline for customer use. The refrigeration potential inherent in such liquid natural gas is large and it is generally desirable that this refrigeration potential be utilized rather than wasted. Accordingly, it is an object of the present invention to provide an efficient method of recovering a substantial portion of the refrigeration potential inherent in such liquid natural gas and of utilizing such refrigeration potential to liquefy a second gas, such as nitrogen, with a reduced power requirement.

To this end the invention comprises:

A method of liquefying a gas utilizing a liquid refrigerant of a type having a boiling point lower than the critical temperature of said gas, said method comprising (a) introducing a main stream of said gas at a selected pressure,-

(b) liquefying said gas by indirect heat exchange with said liquid refrigerant,

(c.) expanding saidliqueed gas in a plurality of successive stages to form a plurality of individual gaseous .recycle streams, each recycle stream being of lesser (g) and feeding said combined recycle streams at said selected pressure into said main stream prior to its liquefaction in said step (c) Further understanding of the various aspects of the present invention will be facilitated by reference to the raccompanying flow sheet, the specific arrangement illustrated being provided by way of example only, and the scope of the invention being defined by the appended claims.

In the drawing, a liow sheet of an arrangement according to the invention is shown, the gas flow being depicted as indicatedby the legend.

Referring now to the flow sheet, gaseous nitrogen from any convenient source enters the system via a conduit 2 andisfcompressed in a three-stage compressor 4 to a pressure of approximately 340 p.s.i.a. (pounds per square inch, absolute), after which the heat of compression is removed by a water aftercooler 6 and a Freon aftercooler ICC 8. The cooled nitrogen gas then passes through an oil separator 10 and oil adsorbers 12 to remove any entrained oil and next passes through a series of three heat exchangers 14, 16 and 18 respectively. In heat exchanger 14 the nitrogen is cooled down to a temperature of approximately 44 C. by relatively high pressure natural gas vapor in coil 20. In heat exchanger 16 the nitrogen is further cooled to a temperature of approximately 104 C. by liquid natural gas vaporizing in coil 22. In heat exchanger 18 the nitrogen gas is cooled to a temperature of approximately 129 C. by liquid natural gas being warmed (but not vaporized in coil 24. The chilled gaseous nitrogen then passes from heat exchanger 18 through a conduit 26 to a heat exchanger 28 where it is cooled to its dew point (approximately 155 C. at this pressure of slightly below 340 p.s.i.a.) against low pressure liquid natural gas vaporizing in coil 30. The nitrogen then leaves heat exchanger 28 via a 4conduit 32, mixes at point 34 with a recycle stream (to be described presently) in a conduit 112 and passes through another heat exchanger 36 where it is liquefied against low pressure liquid natural gas vaporizing in coil 38.

The liquid nitrogen leaves heat exchanger 36, via conduit 40, at a temperature of 155 C. and a pressure of approximately 335 p.s.i.a. The liquid nitrogen is conducted by conduit 40 to a cold exchanger 42 where it is sub-cooled by iirst, second, and third recycle streams in coils 44, 46, and 48 respectively. The liquid nitrogen is next expanded through a valve 50 into a separator 52. Gaseous nitrogen ilashed olf in separator 52, at a temperature of approximately 172 C. and a pressure of about 123 p.s.i.a., becomes the irst recycle stream.

Liquid nitrogen from separator 52, also at a temperature of 172 C. and a pressure of 123 p.s.i.a., enters a second cold exchanger 54 where it is cooled against the second and third recycle streams in coils 56 and 58 respectively. The liquid nitrogen after being cooled in cold exchanger 54 is expanded through a valve 60 into a separator 62, the gaseous nitrogen flashed off in separator 62 becoming the second recycle stream at a temperature of about 184 C. and a pressure of about 47 p.s.i.a.

The liquid nitrogen from separator 62, also at a temperature of 184 C. and a pressure of. 47 p.s.i.a., enters a third cold exchanger 64, where it is cooled against the third recycle stream in coil 66 and is then expanded through a valve 68 into a separator 70. The gaseous nitrogen flashed olf in separator 70, now at a temperature of about 193 C. and a pressure of about 20 p.s.i.a., becomes the third recycle stream.

The liquid nitrogen from separator 70, also at a ternperature of 193 C. anda pressure 20 p.s.i.a. (i.e. just slightly above atmospheric) leaves the system via a conduit and liquid nitrogen lilters 82.

Reference is next made to the three vrecycle streams mentioned above. The third recycle stream (being gaseous nitrogen at a pressure of about 20 p.s.i.a.) passes as mentioned through coils 66, 58, and 48 in heat exchangers 64, 54, and 42 respectively to cool incoming liquid nitrogen. By the time the third recycle stream leaves cold exchanger 42, it has been warmed to a temperature of about C., i.e., the temperature of the incoming liquid nitrogen in conduit 40. The second and first recycle streams, after passing through coils 46 and 44 of cold exchanger 42, are also warmed to a temperature of about 155 C.

The third recycle stream, after leaving coil 48, passes through a conduit 84 to a compressor 86 where it is compressed to a pressure of about 47 p.s.i.a., i.e., the pressure of the second recycle stream. The heat imparted to the third recycle stream by this compressionis removed by low pressure liquid natural gas vaporizing in a coil 88 of a heat exchanger 90. The third recycle stream leaves heat exchanger 90 via conduit 92 at a temperature of about 155 C. (i.e. the temperature of the second recycle stream) and mixes at point 94 with the second recycle stream leaving cold exchanger 42.

The combined second and third recycle streams are then compressed in a compressor 96 to a temperature of approximately 123 p.s.i.a. (i.e. the pressure of the first recycle stream) and the heat of compression is removed in heat exchanger 98 by means of vaporizing low pressure liquid natural gas in coil 100. The combined second and third recycle streams leave heat exchanger 98, via conduit 102, at a temperature of approximately 155 C. (i.e. the temperature of the first recycle stream) and mix at point 104 with the first recycle stream. The three combined recycle streams are then compressed in compressor 106 to a pressure of approximately 340 p.s.i.a. and the heat of compression is removed in heat exchanger 108 by low pressure liquid natural gas vaporizing in coil 110. Nitrogen gas leaves heat exchanger 108 at a temperature of about 155 C., this being its dew point at the pressure of 340 p.s.i.a., and is conducted via the conduit 112 to mix with the main feed stream at point 34 as previously mentioned.

In brief summary of the nitrogen circuit, the flow through conduit 2, heat exchangers 14, 16 and 18, conduit 26, heat exchangers 28 and 36, conduit 40, cold exchangers 42, 54 and 64, and output conduit 80, may be thought of as the main stream. The nitrogen subtracted from the main stream at the three separators 52, 62 and 70 in the form of three recycle streams is added back to the main stream at point 34. It is evident that the gaseous .nitrogen input to the system via conduit 2 is equal in mass to the liquid nitrogen withdrawn from the system via output conduit 80.

Referring now in more detail to the natural gas circuit, liquid natural gas is supplied, usually from low pressure storage, and at a temperature of 158 C., via conduits 114 and 116, to pumps 118 and 120 respectively. In pump 120 the liquid natural gas is pumped to a pressure of just a few p.s.i. above atmospheric pressure. After leaving pump 120, the liquid natural gas passes through a conduit 122 and divides into five streams, each stream passing through one of the coils 30, 38, 88, 100, and 110 of the five respective heat exchangers 28, 36, 90, 98, and 108. As mentioned, the liquid natural gas vaporizes in each of these five coils to cool the nitrogen flowing through the associated heat exchangers. A relatively low pressure is required in these coils (just high enough to .move the liquid natural gas through the five heat exchangers referred to and to prevent air leakage into the system) because, if the pressure were too high, the liquid natural gas would vaporize in the heat exchangers at too high a temperature, possibly above the critical temperature for nitrogen of 147.1 C. In such case, the ve heat exchangers referred to would not cool the gaseous nitrogen sufficiently for it to condense.

Natural gas in vapor form, after leaving the five coils 30, 38, 88, 100 and 110, recombincs in conduit 124. The natural gas is then conducted to a compressor 126 where it is compressed to a higher pressure and then mixed with vhigh pressure natural gas vapor at point 128, as will be described shortly.

Liquid natural gas coming in from a storage via conduit 114 is pumped by pump 118 to a pressure of about 300 p.s.i. and leaves pump 118 at a temperature of 153 C. This high pressure liquid natural gas then passes through coil 24 of heat exchanger 18, where it is warmed (but not vaporized) against nitrogen gas passing through heat exchanger 18. This warmed liquid natural gas, now at a temperature of 107 C. next passes through coil 22 of heat exchanger 16 where it is vaporized to cool nitrogen gas passing through heat exchanger 16. Natural gas vapor, still at a temperature of 107 C., leaves coil 22 and passes through 4coil 20 of heat exchanger 14 where it is warmed to a temperature of +2 C. against nitrogen gas passing through heat exchanger 14. This warmed natural gas vapor, now at a pressure of about 294 p.s.i.a., travels via a conduit to mix with gas from compressor 126 at point 128. The combined natural gas vapor streams at point 128 may be passed through an output compressor 132 for further compression depending on customer requirements, and the natural gas vapor leaves the system via a conduit 134. In general, customer requirements demand that natural gas in vapor form leaving the system be at a relatively high pressure. Since considerably less power is required in pumping a liquid to a high pressure as opposed to compressing a gas to a high pressure, a considerable saving in input power to the system is achieved by pumping liquid natural gas in conduit 114 to a high pressure before vaporizing it against nitrogen gas in heat exchangers 14, 16 and 18. The liquid natural gas vaporized in the five heat exchangers 28, 36, 90, 98 and 108 must of necessity remain at a low pressure for reasons as mentioned.

It will further be observed that input power to the natural gas output compressor 132 is reduced because of the presence of the water and Freon aftercoolers 6 and 8 respectively. These aftercoolers reduce the temperature of the nitrogen gas passing through heat exchangers 14, 16 and 18, and therefore the temperature of the natural gas vapor leaving this series of heat exchangers via conduit 130 is reduced. Since less power is required to compress cool natural gas vapor than is required to compress warm natural gas vapor, this arrangement as mentioned reduces input power to compressor 132.

It should be noted that although this description has dealt with an arrangement for liquefying a particular gas (nitrogen) utilizing a particular refrigerant (liquid natural gas), other gases (such as oxygen) could be used, and other refrigerants could be utilized, providing that the boiling point of the liquid refrigerant at a convenient pressure is lower than the critical temperature of the gas.

In this specification, gasifying and vaporizing are used interchangeably.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of our invention as defined in the appended claims.

We claim:

1. A method of liquefying a gas from thegroup consisting of nitrogen and oxygen utilizing a liquid refrigerant of a type having a boiling point, at a convenient pressure, lower than the critical temperature of said gas, said method comprising (a) introducing a main stream of said gas at a selected pressure,

(b) liquefying said gas by indirect heat exchange with said liquid refrigerant,

(c) expanding a portion of said liquefied gas in a plurality of successive stages to form a plurality of individual gaseous recycle streams, each recycle stream being of lesser pressure than the preceding recycle stream,

(d) compressing each recycle stream to the pressure of the preceding recycle stream,

(e) removing the heat of compression generated by said step (d) by indirect heat exchange between such compressed recycle stream and said liquid refrigerant,

(f) combining such compressed and cooled recycle stream with the preceding recycle stream prior to compression of the latter,

(g) and feeding said combined recycle streams at said selected pressure into said main stream prior to its liquefaction in said step (b).

2. A method according to claim 1 wherein said liquid refrigerant is vaporized in each of said steps (b) and (e) at said convenient pressure.

3. A method according to claim 1, wherein said liqueed gas, upstream of each said expansion forming a recycle stream, is cooled by indirect heat exchange with such recycle stream and is further cooled by indirect heat exchange with any succeeding recycle stream.

4. A method according to claim 1 including (h) introducing a stream of said liquid refrigerant at a low pressure for use in said steps (b) and (e),

(i) introducing a second stream of said liquid refrigerant at high pressure,

(j) cooling said main gas stream, prior to said step (b),

by indirect heat exchange with said second high pressure liquid refrigerant and vaporizing the latter in the process,

(k) compressing the low pressure refrigerant vapor resulting from said steps (b) and (e) to said high pressure and combining it with high pressure refrigerant vapor resulting from said step (j).

5. A method according to claim 1 including (h) introducing a stream of said liquid refrigerant at a low pressure for use in said steps (b) and (e),

(i) introducing a second stream of said liquid refrigerant at high pressure,

(j) cooling said main gas stream, prior to said step (b),

by indirect heat exchange with said second high pressure liquid refrigerant and vaporizing the latter in the process,

(k) compressing the low pressure refrigerant vapor resulting from said steps (b) and (e) to said high pressure and combining it with high pressure refrigerant vapor resulting from said step (j),

(l) said gas being nitrogen and said liquid refrigerant being liquid natural gas.

References Cited UNITED STATES PATENTS 2,896,414 9/1955 Tung 62-23 XR 3,058,314 10/1962 Gardner 62-40 XR 3,092,976 6/ 1963 Tafreshi 62-9 XR 3,160,489 12/1964 Brocoff et al. 62-23 XR 3,257,813 6/1966 Tafreshi 62-23 20 WILBUR L. BAsCoMB, JR., Primary Examiner.

V. W. PRETKA, Assistant Examiner. 

1. A METHOD OF LIQUEFYING A GAS FROM THE GROUP CONSISTING OF NITROGEN AND OXYGEN UTILIZING A LIQUID REFRIGERANT OF A TYPE HAVING A BOILING POINT, AT A CONVENIENT PRESSURE, LOWER THAN THE CRITICAL TEMPERATURE OF SAID GAS, SAID METHOD COMPRISING (A) INTRODUCING A MAIN STREAM OF SAID GAS AT A SELECTED PRESSURE, (B) LIQUEFYING SAID GAS BY INDIRECT HEAT EXCHANGE WITH SAID LIQUID REFRIGERANT. (C) EXPANDING A PORTION OF SAID LIQUEFIED GAS IN A PLURALITY OF SUCCESSIVE STAGES TO FORM A PLURALITY OF INDIVIDUAL GASEOUS RECYCLE STREAMS, EACH RECYCLE STREAM BEING OF LESSER PRESSURE THAN THE PRECEEDING RECYCLE STREAM, (D) COMPRESSING EACH RECYCLE STREAM TO THE PRESSURE OF (E) REMOVING THE HEAT OF COMPRESSION GENERATED BY SAID STEP (D) BY INDIRECT HEAT EXCHANGE BETWEEN SUCH COMPRESSED RECYCLE STREAM AND SAID LIQUID REFRIGERANT, (F) COMBINING SUCH COMPRESSED AND COOLED RECYCLE STREAM WITH THE PRECEDING RECYCLE STREAM PRIOR TO COMPRESSION OF THE LATTER. (G) AND FEEDING SAID COMBINED RECYCLE STREAMS AT SAID SELECTED PRESSURE INTO SAID MAIN STREAM PRIOR TO ITS LIQUEFACTION IN SAID STEP (B). 